Microwave Absorption Properties Of Nd₂Co₁₇ Materials With Shape Anisotropy And Heterostructured Fe₃O₄ Based Materials

In recent years,with the rapid development of communication technology and radar systems,electromagnetic interference and radiation pollution have become increasingly serious.It is urgent to design and develop absorbing materials with excellent microwave absorption performance.Compared to dielectric loss materials,magnetic loss materials have advantages such as simple preparation and low cost,and have irreplaceable advantages in some practical fields.However,due to the Snoek’s limit,traditional magnetic loss materials are difficult to achieve high permeability in the high-frequency band,which will limit their development and application in the high-frequency field.Usually,for magnetic loss absorbing materials,there are two main approaches and methods to enhance microwave absorption performance.One is to bring in a strong anisotropic field by adjusting the shape and structure of magnetic loss materials,thereby breaking the Snoek’s limit and achieving high permeability at high frequencies.The second is to construct magnetic/dielectric composites with multiple hierarchical structures by introducing dielectric loss materials to form multiple heterogeneous interfaces,thereby enhancing interface polarization and achieving better impedance matching.One of these materials is the flake-like magnetic loss material with strong planar anisotropy,which can easily break’s the Snoek’s limit.Therefore,in this work,based on the flake-like magnetic loss material,two perspectives of further breaking the Snoek’s limit and constructing a multiple heterogeneous structure are used to study and design the absorbing material with excellent microwave absorption performance.First,in this paper,micron Nd₂Co₁₇ flakes were prepared by arc-melting and ball-milling methods.The morphology,lattice structure,and magnetic and microwave absorption properties of Nd₂Co₁₇ flakes with different ball-milling time were investigated.The increasing of ball-milling time led to the increase of aspect ratio and higher coercivity of Nd₂Co₁₇ flakes,which helps to suppress the eddy current effect,increase permeability,and spatial polarization.The minimum RL value of-10.83 d B at the thin thickness of1.75 mm was obtained for Nd₂Co₁₇/paraffin composite with a ball milling time of 6 hours possesses the largest shape anisotropy.Besides,the Nd₂Co₁₇/paraffin composite with a ball milling time of 4 hours exhibited a relatively thin matched minimum thickness of 4.28 mm at RL=-20 d B.Meanwhile,in a wider thickness range,Nd₂Co₁₇/paraffin composites with a ball milling time of 2 hours show the best microwave absorption performance,and the reflection loss reached-58.61 d B at 5.41 mm.Second,the Fe₃O₄@C@MoS₂ ternary layered composites were prepared by a three-step method.Meanwhile,the morphology and microstructure of the prepared Fe₃O₄@C@MoS₂ composites and its precursorsα-Fe₂O₃ and Fe₃O₄@C were investigated.The results of electromagnetic parameter measurements and calculations show that the Fe₃O₄ nanosheets have large shape anisotropy,which can effectively suppress the eddy current effect and generate a strong natural resonance.In addition,the C and MoS₂ cladding layers bring multiple heterogeneous interfaces and introduce enhanced interfacial polarization for the composites.Meanwhile,the large number of defects on the surface of MoS₂ nanoflowers contributes to the enhanced dipole polarization of the composites.The combination of multiple magnetic and dielectric losses results in Fe₃O₄@C@MoS₂ composites with excellent microwave absorption properties:the minimum reflection loss is as high as-64.30 d B at a thickness of 2.24 mm,the maximum effective absorption bandwidth is as high as 6.39 GHz at a thickness of 1.89 mm（11.55-17.94GHz）,covering almost the entire Ku-band.Finally,the prepared Fe₃O₄@C@MoS₂ composite and its precursor were simulated using radar cross section（RCS）.The results show that the Fe₃O₄@C@MoS₂ composites have the smallest scattering signal.Meanwhile,its RCS attenuation value was as high as 31.90 d B m² at the incidence angleθ=0°,which exhibited the strongest radar wave attenuation characteristics.In addition,the electromagnetic parameters of Fe₃O₄@C@MoS₂ composites with different paraffin mass fractions were tested,the precursor Fe₃O₄@C was introduced,and the multilayer structure and composition were designed by the particle swarm optimization（PSO）algorithm.The results show that the sample（total thickness of 6.64 mm）with the combination of Fe₃O₄@C in the first layer of 2.29 mm and Fe₃O₄@C@MoS₂ composites with 40%paraffin mass fraction in the second layer of 4.35 mm has an ultra-wide effective absorption bandwidth of 11.00 GHz,which far exceeds the maximum absorption bandwidth of each material when it is a single layer.